Transcript

Robyn Williams: Do you remember when physics was all over? Finished. Complete. That was over a hundred years ago, and happens regularly – like The End of Politics, The End of Novels, and even The End of Radio.. Can you imagine?

Well physics is in a new spring what with Gravitational Waves, just found, Higgs Bosons, and celebs like Brian Cox, Stephen Hawking, Paul Davies, Lisa Randall, and Helen Czerski able to fill stadiums like rock stars.

And now we have a new book about adventures in physics. It’s actually written for laypeople, and a good crib for students. One of the authors is Ross Barrett, who was with the Defence Science Organisation in Adelaide. Here he tells how the book came to be.

Ross Barrett: The idea for this book began when I joined up with an old friend and former colleague, Pier Paolo Delsanto, to tour for a week through Provence in France. He had recently retired from the Polytechnic University of Turin. We had worked together on a theory of nuclear scattering in the seventies and eighties, but since then we had both changed to other areas of physics and fallen out of touch.

“Wouldn’t it be great to work together again on a joint project” he suggested, and I wholeheartedly agreed. “Let’s write a physics book for non-physicists,” he said, “with little, or no mathematics.” I could see the value of such a book in helping overcome a perception, held by many in our community, that physics is too hard, boring, and irrelevant. We both believe that physics, far from being dry, can be and should be made, beautiful, inspiring and enjoyable. We approached Angelo Tartaglia, a cosmologist also at the Turin Polytechnic, to join us in our endeavour.

Four important components of modern physics are Quantum Mechanics, the Special and General Theories of Relativity, Particle Physics and Cosmology. These all had to be included in our book. However, here we hit the first hurdle. Physics, as a science, is built up one brick upon another. To explain modern physics one needs a knowledge of what has gone before. We decided to begin our story with Galileo, who was the first to use experiment and measurement to test out physical ideas.

As the process of measurement became more exact, particularly with the invention of accurate clocks, the development of Mechanics by Newton, and later formulations by Lagrange and Hamilton, became feasible, and at the end of the Nineteenth Century, classical mechanics was capable, in principle, of accurately predicting the motion of bodies on earth and in space.

Simultaneously, investigations into the nature of light were taking place, and again Newton made a major contribution. His proposition that rays of light were composed of streams of particles formed the basis of much of his work. Useful as this ray approach was to the description of a vast amount of physical data, it became clear by the 19th Century that there were optical phenomena that could not be explained by a corpuscular theory of light.

A definitive experiment was performed in 1803 by Thomas Young, who allowed light to fall on two closely spaced slits. When the light passing through these two slits was allowed to fall on a screen, a pattern of bright and dark fringes was observed. This “interference” is analogous to the interference of ripples on a pond when stones are thrown into the water, and is a characteristic that occurs whenever two trains of waves pass through each other. Newton’s corpuscular theory of light was forced by experimental evidence to make way for a wave theory.

By the middle of the 19th Century two other areas of physics, electricity and magnetism, had been found to be different aspects of the same phenomenon, which was dubbed electromagnetism. Scottish physicist, James Clerk Maxwell, produced a theory of electromagnetism that gathered the existing knowledge together and explained the nature of electromagnetic radiation. He correctly interpreted light as a form of electromagnetic radiation. Newton’s corpuscular theory of light was now not only dead, but well and truly buried.

And so it was that at the end of the 19th Century, physics had reached a stage in its development when all of the outstanding physical problems appeared to have been satisfactorily resolved. The fields of mechanics, optics, and also thermodynamics, were surely all done and dusted, and the jewel in the crown of classical physics, the unification by Maxwell of electricity and magnetism into the theory of electromagnetism, had rounded off the 19th Century for physicists in a glow of self-approbation. So enthused was Lord Kelvin that he is said to have proclaimed to the British Association for the Advancement of Science in 1900 that “there is nothing new to be discovered in physics now. All that remains is more and more precise measurement.”

Big mistake! Within a few years so many new discoveries had been made, and new ideas found to explain them, that physics was shaken to its foundations. Firstly, classical mechanics and Maxwell’s theory of electromagnetism were found to be mutually contradictory when used to predict the results of experiments carried out by two experimenters moving with respect to each other. They couldn’t both be right.

This problem was not resolved until Einstein, a patent clerk in Berne, Switzerland, put forward his Special Theory of Relativity. In this theory, clocks run slower in a moving laboratory compared with one at rest, distances shrink in the direction of motion, and events which appear simultaneous to one observer are seen to occur at different times by another observer moving with respect to the first. The very nature of time and space is challenged, and a new unified space-time is proposed. Energy and mass are found to be related and convertible from one to the other according to the relationship: Energy equals mass times the velocity of light squared. Such a device as an atomic bomb had become conceivable.

Ten years after his Special Theory, Einstein produced his General Theory of Relativity, which is really a theory of gravity. Unlike Newton’s theory of gravity, which involves action at a distance produced by a gravitational field, Einstein interpreted gravity as a consequence of curvature in space-time induced by the presence of a mass. This curvature affects the path of the mass through space-time, and produces the effects of gravity that we observe every day.

Mathematically the General Theory of Relativity is extremely difficult. However, the theory predicts the existence of black holes and gravitational waves, and is the theory underpinning modern cosmology.

Meanwhile, in the laboratory, other physicists observed the spectrum of electromagnetic radiation emitted by black bodies heated in a furnace, and found that it cannot be explained by classical theories. Max Planck, a German physicist, offered an ad hoc explanation whereby the radiation is emitted in discrete packets, or quanta, whose energy is proportional to the frequency of the radiation. In other experiments, it was discovered that ultraviolet light incident on a metal can eject electrons with an energy also related to the radiation frequency. Einstein explained this phenomenon by returning to a corpuscular theory and interpreting light as a stream of particles, or photons.

So, in one set of experiments we have light behaving like a wave and diffracting when it passes through a slit, and in another it behaves like a stream of particles. The shade of Newton must surely have chuckled quietly and rubbed his hands with glee. To add to the confusion, other particles, such as electrons, were also found to exhibit wave-like properties when passing through slits.

The mutual reconciliation of the wave and particle theories occupied some of the greatest scientists of the 20th Century and ultimately led to the development of Quantum Mechanics. Quantum Mechanics provides an unprecedented challenge to our capacity to understand, or even to define what understanding means. Richard Feynman, Nobel laureate for his fundamental contributions to this area, once famously remarked: “I think I can safely say that nobody understands Quantum Mechanics”.

In Quantum Mechanics the wave-particle dilemma is approached by associating a wave function with each particle. This wave function is quite unlike a real wave, but is a mathematical device used to calculate the probability of finding a particle in a particular place at a specified time. In the two slit experiment we mentioned earlier, the wave function predicts that more photons will reach the bright fringes on the screen than the dark ones.

The statistical nature of Quantum Mechanics was not accepted by Einstein, who quipped: “God does not play dice with the universe.” However, the predictions of Quantum Mechanics have been tested countless times since its inception, and it is used routinely in all calculations of the properties and interactions of atoms, molecules and sub-atomic particles. In Quantum Electrodynamics, which describes the interaction of electrons with photons, predictions made with this theory have been verified to an accuracy of ten parts in a billion. This is equivalent to measuring the distance from Sydney to Perth to an accuracy of 4 cm.

One of the many bizarre predictions of Quantum Mechanics is “entanglement”. If two particles are produced simultaneously, their wave functions may be coupled in such a way that their total angular momentum is fixed. Then if one particle impinges on a detector that measures its angular momentum, we immediately know the other particle’s angular momentum, no matter where in the universe that particle is now located. What is even stranger is that neither particle’s angular momentum is defined in any way until the measurement has been made, and then both angular momenta are specified simultaneously. We all, physicists included, might dismiss these predictions as nonsense, as Einstein did, but quantum entanglement has been demonstrated in the laboratory and is currently being investigated for application to computer technology.

We have discussed earlier the contradictions between classical mechanics and Maxwell’s theory of electromagnetism at the end of the 19th Century. Now as we progress into the twenty-first century, we find ourselves in a very similar situation. On close inspection, the two theories that represent the pinnacles of 20th Century physics appear themselves to be in contradiction.

Quantum Mechanics is a probabilistic theory, where only the statistical likelihood of an event can be calculated. Relativity is a deterministic theory where events can be foretold precisely. In fact, Relativity challenges the classical nature of time, with past, present and future depending on the perspective of the observer. Events which are simultaneous for one observer may not be simultaneous for another.

And yet QM tells us that if two particles are entangled, and a measurement is made of the angular momentum of one of them, we know simultaneously the angular momentum of the other. Simultaneously, but from whose point of view?

So where does physics go from here? We cannot say. Perhaps somewhere, labouring in a patent office in Berne, or commencing a Ph.D. course at one of the world’s numerous universities, there is a new Einstein whose genius will resolve these enigmas, reconcile the two great theories of the 20th Century, and construct an effective quantum theory of gravity. Then again, many decades of slow and painstaking work by thousands of dedicated researchers may be necessary before any insight is gained. Currently several different approaches to this problem are being explored, but so much more still needs to be done.

One thing is plain: the science of physics is anything but dry, boring and irrelevant, and if our book makes this clear, it will have served its purpose.

Robyn Williams: And the book is called Physics: The Ultimate Adventure. And that was author Ross Barrett in Adelaide. The publisher is Springer. Suitable for students and clever laypeople like you and me.

Next week, Christianity and science – how can they coexist more serenely? We move to Canberra for an answer. I’m Robyn Williams.